A multispeed load-shiftable transmission generally comprises gearing establishing selected paths between an input shaft (which may be counted to an engine of an automotive vehicle) and an output shaft (which may be connected to a load such as the driving wheels of the vehicle). Generally speaking, and as described in the aforementioned patents, "speed" or "gear" selection is effected by selection among the several transmission ratios, by the hydraulic actuation of certain friction devices, namely, hydraulic clutches and/or brakes. These friction devices serve to couple parts of the transmission together to establish one or another gear train. For example, when the friction device is a brake, it may immobilize a free-running part of a planetary-gear speed-reducing gear train to establish one transmission ratio, or "speed" or "gear". A clutch may be used to couple a particular gear train to the input or output shaft. When friction devices of this type are used, speed selection can be effected without decoupling the output shaft from the load and hence, the transmission is said to be shiftable under load or load-shiftable.
In the latest-issued of the aforementioned patents, a hydraulic system is disclosed which effects efficient hydraulic actuation of a pair of concurrently operating clutches or brakes, i.e. hydraulic devices or friction devices of the aforedescribed type. While a detailed description of that system is not necessary for an understanding of the present improvement, since that patent and U.S. Pat. No. 3,610,070 are hereby incorporated by reference in their entirety for all subject matter relating to the transmission and hydraulic connections which have not been described, some definitions appear to be desirable.
When reference is made herein, therefore, to a 1-2 shift valve or a 2-3 shift valve, it will be understood that such reference is intended to mean a slide valve, preferably of the spool type, which is hydraulically actuated to shift the gear transmission between its first and second speeds and between its second and third speeds respectively, either for upshifting or downshifting. The actuation of these valves may be hydraulic, i.e. by pressurization of these valves from one end, against the force of respective restoring springs, by any convenient means.
When reference is made herein to a "damper" it is to be understood that such a device is intended to permit controlled increase in pressure and to prevent the generation of pressure shocks (jolts). Such a damper may be a spring-loaded pressure accumulator and, unless otherwise stated, may be any convenient pressure accumulator of conventional design.
When reference is made to "load-modulated" pressure, it is intended to refer to a hydraulic pressure which is a function of the load and hence a torque applied at the output shift. This pressure can be considered to be throttled supply pressure and hence such terminology is often used.
U.S. Pat. No. 3,610,070 describes a system in which two fluid-operated clutches or brakes, acting as drive-establishing means for a multi-speed transmission, are concurrently operated under the control of a set of valves in response to manually or automatically generated speed-selection commands.
The system specifically described in this patent comprises, as a first and a second drive-establishing means, a unidirectionally effective and a bidirectionally effective brake which are both filled with a high-pressure hydraulic fluid upon an upshift from a low-speed position (first gear) into an intermediate speed position (second gear), the engagement of both brakes being moderated by a control valve which in its normal position admits fluid to them at a relatively high rate but which, upon incipient pressure build-up in the unidirectionally effective first brake, is moved by fluid feedback into a off-normal position resulting in throttled flow.
After the pressure in the first brake (which at this stage is directly connected to the second brake) has reached a predetermined level, the normal position of the control valve is established by a force-increasing action of that fluid pressure upon a piston member opposing the off normal displacement of this valve. Thus the two brakes are under full supply pressure as soon as the upshift into the second gear is completed.
A further upshift into a high-speed position (third gear) requires the release of the bidirectionally effective second brake. This release is brought about by the draining of the feeder line of that brake via the normal position of the control valve and a 2-3 shift valve in tandem therewith.
In the event of a subsequent downshift to the intermediate high-speed position (second gear), the first brake is already actuated so that only the second brake need be supplied with fluid. This occurs in response to a return of the 2-3 shift valve from an upshifting to a downshifting position with no further intervention of the control valve in tandem therewith. If the upshift into third gear occurs before the second gear position is fully established, i.e. with the control valve still in its off-normal throttling position, the draining of the bidirectionally effective brake cannot take place immediately but must await the restoration of the control valve to normal. The resulting delay may lead to concurrent actuation of the two brakes and of a third friction device, namely, a clutch effective in third gear only whereby, with three drive-establishing means acting upon a common movable element, i.e. the output shift, the system jams for brief periods and parts thereof may be subjected to excess stress.
To avoid this disadvantage I, together with others, proposed the improvement which has been fully described in U.S. Pat. No. 3,938,410 whereby drain means is provided for the second drive-establishing means, specifically the bidirectionally effective brake, effective in the blocking position of 2-3 shift valve independently of the position of the associated control valve.
In accordance with this improved control system, the feed line for the second friction device or brake, is split into two sections, a first section extending from the control valve to a first port of another valve cylinder and a second section extending from a second port of that cylinder to the second friction device. In the unblocking position of the 2-3 shift valve, a piston in this valve cylinder interconnects the two ports; in the blocking position, it obstructs the two ports and connects the two ports to a drain.
The piston and cylinder of this improved system may form part of the 2-3 shift valve or may be directly controlled thereby through the intermediary of a discharge port which communicates with the fluid-supply channel in the blocking position of this valve and applying fluid pressure, counteracting a restoring force, to the piston of the drain valve. In the latter case, the same discharge port advantageously feeds the third friction device, namely, the aforementioned clutch.
U.S. Pat. No. 3,938,410 thus describes a hydraulic actuating system which affords an excellent shift transition using two parallel-actuated clutches with the same pressure buildup although optimum operation is not, however, obtainable therewith because the pressure buildup in the second clutch or brake takes place with a small time lag of about 0.1-0.3 seconds, although the subsequent pressure increase is completely satisfactory.
The lag results because the control edge of the valve piston 151c (see the drawing of this patent) opens only when the valve 150 is shifted into its control position and permits the unobstructed completely equal pressurization of the first and second friction devices B1F and B1via the passages 166, 166c and 175. Further the orifice in the throttle 216a cannot be made larger since this orifice determines the quality of the downshift from third speed to second speed. The first friction device BIF, as a result of the lag of the second friction device B1 is subjected to higher thermal loading.
The valve provided in the feeder line of the second friction device operates as a drain valve to avoid detrimental overlap during speed changes and is connected with the feeder line of the further friction clutch or with the 2-3 shift valve.